\u3cem\u3eAPOE\u3c/em\u3e and Alzheimer’s Disease: Neuroimaging of Metabolic and Cerebrovascular Dysfunction

Apolipoprotein E4 (ApoE4) is the strongest genetic risk factor for late onset Alzheimer’s Disease (AD), and is associated with impairments in cerebral metabolism and cerebrovascular function. A substantial body of literature now points to E4 as a driver of multiple impairments seen in AD, including blunted brain insulin signaling, mismanagement of brain cholesterol and fatty acids, reductions in blood brain barrier (BBB) integrity, and decreased cerebral glucose uptake. Various neuroimaging techniques, in particular positron emission topography (PET) and magnetic resonance imaging (MRI), have been instrumental in characterizing these metabolic and vascular deficits associated with this important AD risk factor. In the current mini-review article, we summarize the known effects of APOE on cerebral metabolism and cerebrovascular function, with a special emphasis on recent findings via neuroimaging approaches

Blood Glucose Levels and Genetic Factors as Predictors of Neurocognitive Outcomes

Alzheimer’s disease (AD), a neurodegenerative disease characterized by progressive cognitive decline, is becoming increasingly prevalent as the population ages. There is no effective treatment for AD, so manipulating modifiable risk factors before clinical symptoms of dementia develop may be the most effective course to prevent, delay, or modify the course of AD. Type II diabetes mellitus (DM), characterized by hyperglycemia and insulin resistance, affects over a quarter of older adults and has been linked with AD, cognitive decline, and brain atrophy. Because DM is preventable and treatable, it represents an intervention target for AD. This dissertation investigated the association between glucose levels and three outcomes – performance on a battery of cognitive tests over time, time to symptom onset of mild cognitive impairment (MCI) or dementia, and cortical thickness in AD-signature regions (i.e., entorhinal cortex, inferior temporal gyrus, parahippocampal gyrus pars triangularis, precuneus, superior frontal and parietal gyri, supramarginal gyrus, and temporal pole). Additionally, it investigated how two genes – apolipoprotein E (APOE) and translocase of the outer mitochondrial membrane (TOMM) 40 – may modify the relationship between glucose and these outcomes. Both the APOE ε4 allele and the TOMM40 very long/very long (VL/VL) genotype (versus the short/short (S/S) genotype) have been associated with increased risk of dementia, cognitive decline, and brain atrophy, and some evidence suggests that these genetic factors may further increase the risk in individuals with DM. This research was conducted using data from the BIOCARD Study (n=349), which enrolled primarily middle-aged individuals, three-quarters of whom had a first-degree relative with dementia. The goal of the study was to examine the early signs and symptoms of AD in a high-risk cohort. For this dissertation, baseline blood glucose level, which was ascertained from a blood draw in a clinical exam, was the primary predictor and was available for 333 participants. APOE and TOMM40 genotypes were also determined from blood samples. Age of MCI or dementia symptom onset was determined through consensus diagnosis. At annual visits, participants completed a neuropsychological test battery, and at baseline and bi-annually, participants completed magnetic resonance imaging (MRI) scans. The first paper in this study used linear mixed effects models (LMMs) and generalized estimating equation (GEE) models to investigate the association between glucose and cognitive performance over time. We used confirmatory factor analysis (CFA) to create one factor based on performance on tests of executive function (the executive function factor) and one factor based on performance on tests of memory (the memory factor). We found that higher baseline glucose was associated with greater decline on the executive function factor score in both LMMs (B=-0.005; 95% CI -0.008, -0.001) and GEE models (B=-0.004; 95% CI -0.007, -0.001) that controlled for age, sex, race, education, depression, and medical conditions (i.e., cardiovascular conditions, hypertension, hypercholesterolemia, traumatic brain injury (TBI)). Interaction analyses using LMMs found that higher glucose was associated with significant decline in executive function score in ε4 carriers (B=-0.013; 95% CI -0.020, -0.006), but not in non-carriers (B=-0.002, 95% CI -0.006, 0.002). In LMMs, higher glucose was also associated with poorer memory factor score over time in subjects with the S/S genotype (B=-0.013; 95% CI -0.024, -0.002), versus the VL/VL genotype (B=0.004; 95% CI -0.003, 0.010). The second paper in this study used Cox proportional hazard models to investigate the association between baseline blood glucose level and time to MCI or dementia symptom onset, as well as the association between the interaction of APOE and glucose and TOMM40 and glucose and time to symptom onset. We did not find an association between glucose level and time to symptom onset; additionally, we did not find that APOE or TOMM40 modified this relationship. In the final paper, linear regression analyses showed that glucose level was associated with reduced cortical thickness in the parahippocampal gyrus (B=-0.002; 95% CI -0.003, -0.0001) and temporal pole (B=-0.002; 95% CI -0.003, -0.0002) in a fully-adjusted model. Additionally, higher glucose levels were associated with thinner measures of the superior parietal gyri (B=-0.001; 95% CI -0.002, 0.0005 vs. B=0.0007; 95% CI 3.05e-6, 0.001) and temporal pole (B=-0.004; 95% CI -0.008, -0.0009 vs. B=-0.0003; 95% CI -0.002, 0.001) in ε4 allele carriers versus non-carriers. This evidence suggests that even in middle-aged, cognitively healthy samples, higher levels of glucose, even in those without DM, can negatively impact cognitive performance and cortical thickness. Maintaining normal blood glucose levels may be important in middle age for reducing risk of cognitive decline, AD, and brain atrophy

Reduced brain activity during a working memory task in middle-aged apolipoprotein E +4 carriers with overweight/obesity

Objective: The apolipoprotein E +4 (APOE +4) allele and midlife obesity are independent risk factors for Alzheimer’s disease (AD). Both of these risk factors are also associated with differences in brain activation, as measured by blood oxygenation level-dependent (BOLD) responses, in the absence of detectable cognitive deficits. Although the presence of these risk factors may influence brain activity during working memory tasks, no study to date has examined whether the presence of the +4 allele explains variation in working memory brain activity while matching for levels of overweight/obesity. The primary aim of this study was to determine whether the presence of the +4 allele is associated with differences in task-functional magnetic resonance imaging (fMRI) brain activation in adults with overweight/obesity. We predicted that +4 carriers would have greater brain activation in regions that support working memory. Methods: This ancillary study included 48 (n = 24 APOE +4 carriers; n = 24 APOE +4 non-carriers), sedentary middle-aged adults (Mean age = 44.63 8.36 years) with overweight/obesity (Mean BMI = 32.43 4.12 kg/m2) who were matched on demographic characteristics. Participants were a subsample enrolled in 12-month randomized clinical trial examining the impact of energy-restricted diet and exercise on cardiovascular health outcomes. Participants completed a n-back working memory task with fMRI, which were completed within one month of the start of the intervention. Participants also underwent pseudo-continuous arterial spin labeling scans, a MRI measure of cerebral blood flow (CBF). Results: Compared to non-+4 carriers with overweight/obesity, +4 carriers with overweight/obesity had lower fMRI brain activity in the middle frontal gyrus, pre and post central gyrus, supramarginal gyrus, superior temporal gyrus, lateral occipital cortex, and angular gyrus (z range = 2.52–3.56) during the n-back working memory task. Differences persisted even when controlling for CBF in these brain regions. Conclusion: These results indicate that presence of the APOE +4 allele in middle-aged adults with overweight/obesity is related to altered brain activity during a working memory paradigm, which may confer risk for accelerated neurocognitive decline in late adulthood. Future research is needed to clarify the clinical implications of these findings in the context of risk for AD.United States Department of Health & Human ServicesNational Institutes of Health (NIH) - USA University of Pittsburgh Clinical and Translational Science Institute (CTSI) 3R01AG060741-03S1 R01 DK095172 R01-HL103646 UL1 TR00185

Early detection of Alzheimer’s disease - Twin study on episodic memory and imaging biomarkers of neuroinflammation and β-amyloid

The disease process of Alzheimer’s disease (AD) causes damage to the brain for several years leading to the development of mild cognitive impairment (MCI) and finally to dementia which interferes with independent living. The early detection of AD disease process is key for the prevention and treatment of disease. The aim of this thesis was to improve the assessment of episodic memory (EM) and cognitive performance with a telephone interview and neuroimaging of early AD. The study population belonged to the older Finnish Twin Cohort study. 2631 twins (856 pairs) participated in the telephone interview (TELE, TICS) during 1999–2007 and 1817 twins (559 pairs) participated in the interview (TELE, TICS, TICS-m) during 2013– 2017. Cognitively discordant twin pairs were asked to participate in more detailed examinations. 11 twin pairs participated in [11C]PBR28 positron emission tomography (PET) imaging measuring neuroinflammation during 2014–2017 and 45 twin pairs participated in [11C]PiB PET imaging measuring β-amyloid (Aβ) deposits during 2005–2017. Twins who had co-twins with dementia (n=101) performed poorer than average in a word list learning test. When using the telephone interview TICS-m, the education‐adjusted classification resulted in a higher proportion of apolipoprotein (APOE) ε4 allele carriers among those identified as having MCI. Twins with poorer EM performance (n=10) had higher cortical [11C]PBR28 uptake compared to their better-performing co-twins. In addition, higher cortical [11C]PiB uptake was associated with poorer EM performance. The results from the telephone interview studies indicate that poorer word list learning performance may be an early marker of dementia risk and that the use of education‐adjustment may increase the accuracy of MCI classification. The twin pair setting controlling for genetic and environmental effects indicated that brain Aβ load and neuroinflammation have a negative association with EM performance.Alzheimerin taudin varhainen havaitseminen – Kaksostutkimus episodisesta muistista ja neuroinflammaation ja β-amyloidin kuvantamisbiomarkkereista Alzheimerin taudin (AT) prosessi vaurioittaa aivoja vuosien ajan ja johtaa lievään kognitiiviseen heikentymiseen (MCI) ja lopulta itsenäistä selviytymistä häiritsevään dementiaan. AT:n dementiaan johtavan prosessin varhainen havaitseminen on avainasemassa ehkäisyn ja hoidon kannalta. Tämän väitöskirjatutkimuksen tavoitteena oli kehittää puhelinhaastattelun käyttöä episodisen muistin (EM) ja muiden tiedonkäsittely- eli kognitiivisten toimintojen arvioimisessa sekä AT:n varhaista kuvantamista. Tutkimusjoukko kuului vanhempaan suomalaisen kaksoskohorttitutkimukseen. 2631 kaksosta (856 paria) osallistui puhelinhaastatteluun (TELE, TICS) 1999–2007 aikana ja 1817 kaksosta (559 paria) osallistui haastatteluun (TELE, TICS, TICS-m) 2013–2017 aikana. Kognitiivisesti diskordantit kaksosparit kutsuttiin tarkempiin jatkotutkimuksiin. 11 kaksosparia osallistui neuroinflammaatiota mittaavaan [11C]PBR28-merkkiaineen positroniemissiotomografia (PET) - kuvaukseen 2014–2017 aikana ja 45 kaksosparia osallistui aivojen β-amyloidikertymää mittaavaan [11C]PiB-merkkiaineen PET-kuvaukseen 2005–2017 aikana. Sellaisten kognitiivisesti normaalien ikääntyneiden kaksosten (n=101), joiden sisaruksella oli dementia, havaittiin suoriutuvan keskimääräistä heikommin sanalistan oppimista mittaavassa testissä. Käytettäessä TICS-m-puhelinhaastattelua koulutuskorjauksen käyttäminen johti siihen, että MCI:tä sairastavien joukossa oli suurempi osuus apolipoproteiini E:n (APOE) ε4-alleelin kantajia. Kaksosilla (n=10), jotka suoriutuivat heikommin EM-testeissä, oli suurempi aivokuoren [11C]PBR28-kertymä verrattuna paremmin suoriutuviin sisaruksiinsa. Myös suurempi aivokuoren [11C]PiB-kertymä oli yhteydessä heikompaan EM-suoritukseen. Puhelinhaastattelujen tulokset viittaavat siihen, että sanalistan oppiminen voi olla dementiariskistä kertova varhainen merkki ja että koulutuskorjauksen käyttö voi lisätä MCI-luokittelun tarkkuutta. Kaksosasetelma, joka kontrolloi geneettisten ja ympäristötekijöiden vaikutusta, osoitti, että aivojen β-amyloidikertymä ja neuroinflammaatio ovat negatiivisessa yhteydessä EM:n toiminnan kanssa

ApoE4 effects on the structural covariance brain networks topology in Mild Cognitive Impairment

The Apolipoprotein E isoform E4 (ApoE4) is consistently associated with an elevated risk of developing late-onset Alzheimer's Disease (AD). However, little is known about his potential genetic modulation on the structural covariance brain networks during prodromal stages like Mild Cognitive Impairment (MCI). The covariance phenomenon is based on the observation that regions correlating in morphometric descriptors are often part of the same brain system. In a first study, I assessed the ApoE4-related changes on the brain network topology in 256 MCI patients, using the regional cortical thickness to define the covariance network. The cross-sectional sample selected from the ADNI database was subdivided into ApoE4-positive (Carriers) and negative (non-Carriers). At the group-level, the results showed a significant decrease in characteristic path length, clustering index, local efficiency, global connectivity, modularity, and increased global efficiency for Carriers compared to non-Carriers. Overall, I found that ApoE4 in MCI shaped the topological organization of cortical thickness covariance networks. In the second project, I investigated the impact of ApoE4 on the single-subject gray matter networks in a sample of 200 MCI from the ADNI database. The patients were classified based on clinical outcome (stable MCI versus converters to AD) and ApoE4 status (Carriers versus non-Carriers). The effects of ApoE4 and disease progression on the network measures at baseline and rate of change were explored. The topological network attributes were correlated with AD biomarkers. The main findings showed that gray matter network topology is affected independently by ApoE4 and the disease progression (to AD) in late-MCI. The network measures alterations showed a more random organization in Carriers compared to non-Carriers. Finally, as additional research, I investigated whether a network-based approach combined with the graph theory is able to detect cerebrovascular reactivity (CVR) changes in MCI. Our findings suggest that this experimental approach is more sensitive to identifying subtle cerebrovascular alterations than the classical experimental designs. This study paves the way for a future investigation on the ApoE4-cerebrovascular interaction effects on the brain networks during AD progression. In summary, my thesis results provide evidence of the value of the structural covariance brain network measures to capture subtle neurodegenerative changes associated with ApoE4 in MCI. Together with other biomarkers, these variables may help predict disease progression, providing additional reliable intermediate phenotypes

Presence of ApoE ε4 Allele Associated with Thinner Frontal Cortex in Middle Age

Abstract. The presence of an ApoE ε4 allele (ε4+) increases the risk of developing Alzheimer's disease (AD). Previous studies support an adverse relationship between ε4+ status and brain structure and function in mild cognitive impairment and AD; in contrast, the presence of an ε2 allele may be protective. Whether these findings reflect disease-related effects or pre-existing endophenotypes, however, remains unclear. The present study examined the influence of ApoE allele status on brain structure solely during middle-age in a large, national sample. Participants were 482 men, ages 51-59, from the Vietnam Era Twin Study of Aging (VETSA). T1-weighted images were used in volumetric segmentation and cortical surface reconstruction methods to measure regional volume and thickness. Primary linear mixed effects models predicted structural measures with ApoE status (ε3/3, ε2/3, ε3/4) and control variables for effects of site, non-independence of twin data, age, and average cranial vault or cortical thickness. Relative to the ε3/3 group, the ε3/4 group demonstrated significantly thinner cortex in superior frontal and left rostral and right caudal midfrontal regions; there were no significant effects of ε4 status on any temporal lobe measures

Neuroimaging advances regarding subjective cognitive decline in preclinical Alzheimer’s disease

Abstract: Subjective cognitive decline (SCD) is regarded as the first clinical manifestation in the Alzheimer’s disease (AD) continuum. Investigating populations with SCD is important for understanding the early pathological mechanisms of AD and identifying SCD-related biomarkers, which are critical for the early detection of AD. With the advent of advanced neuroimaging techniques, such as positron emission tomography (PET) and magnetic resonance imaging (MRI), accumulating evidence has revealed structural and functional brain alterations related to the symptoms of SCD. In this review, we summarize the main imaging features and key findings regarding SCD related to AD, from local and regional data to connectivity-based imaging measures, with the aim of delineating a multimodal imaging signature of SCD due to AD. Additionally, the interaction of SCD with other risk factors for dementia due to AD, such as age and the Apolipoprotein E (ApoE) ɛ4 status, has also been described. Finally, the possible explanations for the inconsistent and heterogeneous neuroimaging findings observed in individuals with SCD are discussed, along with future directions. Overall, the literature reveals a preferential vulnerability of AD signature regions in SCD in the context of AD, supporting the notion that individuals with SCD share a similar pattern of brain alterations with patients with mild cognitive impairment (MCI) and dementia due to AD. We conclude that these neuroimaging techniques, particularly multimodal neuroimaging techniques, have great potential for identifying the underlying pathological alterations associated with SCD. More longitudinal studies with larger sample sizes combined with more advanced imaging modeling approaches such as artificial intelligence are still warranted to establish their clinical utility

Oral health & olfactory function : what can they tell us about cognitive ageing?

The objective of this thesis was to advance our understanding of whether oral health and olfactory function may predict accelerated cognitive ageing. Data from two Swedish study populations and one from the United States were applied to investigate the relationship of oral health and olfactory function with cognitive decline and brain ageing in late life. Study I examined the association of self-reported tooth loss with cognitive decline, and brain volume differences in older adults (n= 2715) from the Swedish National study of Aging and Care-Kungsholmen (SNAC-K). A subsample (n= 394) underwent magnetic resonance imaging (MRI). Tooth loss was associated with a steeper global cognitive decline (β: -0.18, 95% confidence interval [CI]: -0.24 to -0.11). Participants with complete or partial tooth loss had significantly lower total brain volume (β: -28.89, 95% CI: -49.33 to -8.45) and grey matter volume (β: -22.60, 95% CI: -38.26 to -6.94). Thus, tooth loss may be a risk factor for accelerated cognitive ageing. Study II Investigated the effect of poor masticatory ability on cognitive trajectories and dementia risk in 544 cognitively intact adults aged ≥50 from the Swedish Adoption/Twin Study of Aging (SATSA) with 22 years of follow-up. Masticatory ability was assessed using the Eichner Index and categorised according to the number of posterior occlusal zones: A (all four), B (3-1), and C (none). After the age of 65, participants in Eichner category B and C showed an accelerated decline in spatial/fluid abilities compared to those in category A (β: -0.16, 95% CI: -0.30 to -0.03 and β: -0.15, 95% CI: -0.28 to -0.02, respectively). Hence, poor masticatory ability is associated with an accelerated cognitive decline in fluid/spatial abilities. Study III examined whether impaired olfaction is associated with cognitive decline and indicators of neurodegeneration in 380 participants (mean age = 78 years) from the Memory and Aging Project (MAP). Participants with hyposmia (β = −0.03, 95% CI: −0.05 to −0.02) or anosmia (β = −0.13, 95% CI −0.16 to −0.09) had a faster global cognitive decline than those with normal olfaction. Impaired olfaction was related to smaller volumes of primarily the medial temporal cortex (β = −0.38, 95% CI −0.72 to −0.01). Olfactory deficits predict faster cognitive decline and indicate neurodegeneration in older adults. Study IV identified age-related trajectories in episodic memory and odour identification, as well as determinants of the trajectories. 1023 MAP participants were followed for up to 8 years with annual assessments. Three joint trajectories were identified; Class 1- stable performance in both functions; Class 2- stable episodic memory and declining odour identification; and Class 3- decline in both functions. Predictors of class membership were age, sex, APOE ε4 carrier status, cognitive activity, and BMI. Episodic memory and olfactory function often show similar trajectories in ageing, reflecting their shared vulnerability to changes in the medial-temporal lobes. Conclusions: Both poor oral health and olfactory deficits may predict cognitive decline and indicate neurodegeneration in the brain. Poor oral health is associated with accelerated cognitive decline and brain ageing, whereas, olfactory deficits may reflect loss of brain integrity in old age